Portable electronic devices, particularly communication devices, such as two-way portable radios and cellular telephones, have gained considerable market acceptance. In particular, two-way portable radios have been advantageously employed in numerous applications, such as police radios, intra-company communications, and taxi-cab fleets, to name a few.
Other applications in which such battery-powered portable communications devices have found use include applications such as oil fields, coal mines, and other environments in which the atmosphere may contain a high percentage of potentially volatile gases or elements. For example, coal mines may become filled with highly volatile methane gas. Similarly, in oil field applications the atmosphere may be saturated with hydrocarbons, and other volatile gaseous materials. Accordingly, battery-powered communications devices used in these environments must be intrinsically safe; that is, safeguards and precautions must be built in to the device and/or the battery-pack powering the device to assure that no sparks are created when the battery pack and the device are connected or disconnected. Intrinsically safe products require the energy available from a source in any given instant to be limited so as not to allow ignition of any volatile gas in the surrounding atmosphere.
In a battery powered portable electronic device, energy is stored in three ways; heat, magnetic fields and electric fields. Magnetic fields are stored in inductive elements, electric fields are stored in capacitive elements, and heat is built up in the device components, being dissipated via passive radiation. Products intended to be used in the most volatile environments, i.e., 26% diatomic hydrogen in air, must be designed to minimize energy storage in all three media. Since the product poses no threat until it is powered by a battery, there must be a means by which the battery's output is limited to a safe level when the battery is attached. This safety means may be dependent upon the design of the electronic device.
Such is the case with many portable electronic devices intended for intrinsically safe applications. The required safety level dictates that the voltage available to said devices be limited so as not to exceed a threshold safe level. This threshold level is determined by the ignition energy of a volatile gas when an equivalent capacitance is instantaneously discharged in the presence of said volatile gas. In many applications the maximum safe voltage is near the operating threshold voltage of the device. In still other applications the device may need to be redesigned to minimize the capacitance in the device to allow a workable voltage to be applied in a safe manner. In these instances the maximum heat produced in the device is controlled by limiting the current available from the battery since the voltage is already limited to or near the operating threshold. In these cases it is likely that the current limit based on the thermal characteristics of the device is less than that required by the inductance of the device and is usually less than that required by the device to operate normally at otherwise maximum settings. Accordingly, the device may not work properly; to a user, it would appear that the battery had suddenly died, without warning.
Referring now to FIG. 1, there is illustrated therein a prior art circuit 10 including a battery pack 12, and a means to limit current supplied to an electronic device 14. The battery pack 12 comprises at least one battery cell to store electrical energy, battery contacts 16 and 18 to electrically couple the battery pack to the electronic device 14, a pass device such as a N-channel MOSFET 20, a bias means such as a resistor 22 for said pass device 20, a current sense resistor 24, and a switching element such as bipolar transistor 26. Current flows in a circuit from battery pack 12 to the device 14 in the direction of arrow 28 and is returned from the device through pass device 20 and sense resistor 24.
While the current is at or below the threshold safe level, the voltage across the sense resistor 24 is not enough to bias bipolar switching element 26 to conduct. The gate of MOSFET pass device 20 is biased high through resistor 22 and the impedance to current flowing through it from drain to source is minimal. When the current through sense resistor 24 is enough to bias the base-emitter junction of switching element 26 current flows through resistor 22 and through the collector of switching element 26. This removes bias from pass device 20 which increases in impedance significantly. Once the impedance is high enough, the current through it is reduced slightly and therefore the voltage across sense resistor 24 is reduced slightly. This causes an equilibrium to be reached where the current through the pass device is enough to slightly bias switching element 26. When a bipolar device used as switching element 26 the current limiting resistor is chosen by dividing the bias voltage by the intended current limit. For example, for a current limit of 1 amp, divide the bias voltage of 0.65 volts by 1 amps. 0.65/1=0.65 ohms. Therefore the correct sense resistor value to cause this circuit structure to limit at 1 ampere of current is 0.65 ohms.
This circuit works well in limiting the steady state current to a safe level, based on the thermal characteristic of components inside the device. The circuit acts very quickly and has proven to be quite safe. However, in order for the device to perform at maximum settings, it may demand current in excess of such limit, but only for brief periods of time, such that the average value of such excursions is less than that necessary to allow unsafe temperatures inside the device. Thus the need exists for a current limiting means that allows has both a steady state and maximum current limits such that the device can perform as intended and yet remain safe.